Various technical devices are made of metals. These metal surfaces can be affected by corrosion or scaling, both impairing the function of the respective device.
A specific problem is limescale, which occurs especially when hard ground water is warmed up, for example in heat exchangers to supply households and companies with warm water. The limescale reduces the heat transfer through the heat exchanger surface and restricts the flow. After a certain time of use, costly cleaning or even replacement of the heat exchanger is necessary. Various types of heat exchangers and various heat sources, such as district heating, oil or gas burners are used. Plate heat exchangers are an efficient solution to transfer heat from district heating to households. Due to the compactness of these heat exchangers, there can be a significant lateral temperature gradient along the plates. Furthermore, the control mode of these heat exchangers, especially when they directly, without a further reservoir, provide warm water to the somewhat irregular demand of the end users, may lead to frequent and sudden temperature changes.
Repellent coatings have been proposed to avoid or reduce scaling. EP 1129789 B1 teaches the application of silane fluorinated resins to form few μm thick coating films that reduce the adhesion of limestone on heat exchangers used to warm up water. However, the coating has only been tested on test panels and on a camp stove. No data on test duration is disclosed. Thus, no data on long-term stability of the coating against temperature changes and temperature gradients is available. Furthermore, the use of fluorinated resins is, from an environmental point of view, not optimal.
WO 2012083970 A1 teaches a repellent coating against scaling based on silanes and a polydimethyl-siloxane additive. Good adhesion to stainless steel and the application of the coating on a heat exchanger is disclosed, but no data on stability against temperature changes is reported, neither any data on stability against strong lateral temperature gradients. An example of a plate heat exchanger with a length of 1.8 m and a temperature difference between inlet and outlet of 10° C. is disclosed, but the fluid used is oil and not water, and the document does not disclose any limescale repellent or reducing properties.
The distance between inlet and outlet is not disclosed, but for a commercial heat exchanger of the described type and size, it is typically in the range of 1.2 to 1.3 m. That would lead to a lateral temperature gradient of ˜0.008° C./mm
DE 10152853 A1 teaches a coating based on epoxysilanes and blocked isocyanates that can be used as easy-to-clean coating for metal surfaces. No data about stability against temperature changes or temperature gradients is disclosed, and the document does not disclose any limescale repellent or reducing properties.
WO 2012018296 A1 discloses a plate heat exchanger with anti-fouling properties with a low-energy surface comprising silicone oxide, prepared, for example, from a silanterminated polymer or a polysiloxan-urethan resin applied to metal surfaces as, for example, copper or stainless steel. No data about temperature gradients is disclosed. The coating withstands a sudden temperature change by dipping into liquid nitrogen at −196° C. and contact with crude oil, but does on the other hand deteriorate in contact with sea water. No data on temperature gradients is provided. The document does not disclose any limescale repellent or reducing properties.
Thus, there is a need in the art of heat exchangers to provide a surface that effectively reduces limestone scaling, but at the same time is stable towards warm water, temperature changes and temperature gradients and thus is able to operate over longer times under alternating conditions. Even though, according to DE 10152853 A1, coatings based on organically modified, crosslinked polysiloxanes firmly bond to metals like steel and are mechanically and chemically resistant, it has been found, that this does not mean that those coatings in general withstand the conditions present in heat exchangers used to supply end users with warm water.
Another specific problem is the corrosion of Aluminium. Aluminium is technically interesting due to its low specific density and low price compared to other metals. Clear polysiloxane coating films based on organosilanes with a thickness of only a few μm can effectively protect Aluminium from corrosion. The low film thickness makes it easy to stay within construction limits for dimensional stability. Furthermore, the coatings are hardly visible which is advantageous for decorative articles that should keep their metallic look and feel. Various silane-based coating compositions have been proposed, cf. U.S. Pat. No. 6,403,164 B1, WO 2012083970 A1 or DE 10152853 A1. However, any improvement in corrosion protection is of commercial interest as it extends the application area.